There have been many proposals in the prior art to utilize the properties of elastomeric materials in the design of shock absorption devices for vehicle suspensions, the earliest of such proposals concentrating upon utilization of natural rubber elastomers and more recent proposals making use of the advantageous characteristics of modern synthetic elastomeric materials. The prior art proposals have also embraced the use of elastomeric materials in conjunction with metal and other spring devices in so-called composite springs and, as will be explained hereinafter, the present invention also embraces this possibility.
Elastomers have been used for many years as anti-vibration components in the suspension systems of automobiles and other vehicles. Modern automobiles may contain more than a dozen different components such as bushes, mounts, etc. Primary suspension is still usually a conventional steel spring with a hydraulic damper, the system often incorporating an elastomeric top mount. Elastomeric components are widely used in anti-vibration systems in automobiles and other ground vehicles as well as aerospace applications. They can combine low spring rates with low or high damping to attenuate vibration. The ability to attenuate vibrations will in general depend on both the amplitude and frequency of the input vibration. Thus, although vibration isolation or damping may be very successful for one source condition, it may be quite ineffective for another source condition. It is a feature of vehicles that a large number of different vibrations from different sources will often be simultaneously superimposed.
Current design procedures for elastomeric anti-vibration devices are usually a compromise aimed at dealing with the worst of the vibrations. Isolation of vibrations is always preferable if natural frequencies of resonance can be made low enough compared to the input frequencies. This often cannot be ensured when dealing with a spectrum of inputs, especially with conventionally designed anti-vibration mounts. Damping is then required to reduce resonant effects (or impact responses), but usually at the expense of isolation of higher frequencies. Design needs to balance these two functions. In addition, design is usually based on an approximation to linear materials behivor although it is well known that elastomers have non-linear static and dynamic force deflection behivor.
One reason that elastomers have not been more extensively used as primary suspension components is that engineers are generally unfamiliar with non-linear, materials and so designs have tended to minimize non-linear aspects, to the detriment of exploiting the full potential of elastomeric components.
Described in European Patent Application No. 85115879 (Ser. No. EP-A2-0184848) is a composite spring comprising a tubular elastomeric body, preferably of rubber, having a coil spring embedded in and bonded to it; the coil spring is said to control the occurrence of symmetric bulging instability in the elastomeric body under axial load conditions so that this bulging instability occurs sequentially along the length of the body between adjacent coils of the coil spring until it assumes the form of a continuous coil of elastomer. By virtue of this arrangement, the force/deflection characteristic of the composite spring is said to be controllable to provide selected stiff and soft regions in accordance with specific requirements, particularly the provision of a soft load bearing characteristic over an extended deflection range at a predetermined load, with the soft region corresponding to the symmetric bulging of the elastomer body. The application of such a composite spring device to automotive suspensions is further discussed in Rubber Chemistry and Technology, Vol. 59, No. 5, November-December 1986, pp. 740 to 764, in the article entitled "On the role of nonlinearity in the dynamic behavior of rubber components" by J. Harris and A. Stevenson, the inventors of the present invention.
While the force/deflection characteristic described in EP-A2-0184848 abovementioned has attractions for automotive suspension applications, it is our current belief that a composite spring device as described in EP-A2-0184848 may not be reliably and consistently capable of achieving characteristics significantly different from those obtained by much earlier and more recent prior art devices. In this connection, reference may be made to U.S. Pat. No. 1,032,454, issued in 1912 which discloses a rubber body within which there is embedded a helical spring. Other disclosures of composite springs comprising generally cylindrical elastomer bodies having helical springs embedded therein are shown in French Patent No. 34669 (patent of addition to FR-A-559350), issued in 1929, U.S. Pat. Nos. 2,156,580 issued in 1939, 2,605,099 issued in 1952, 2,822,165 issued in 1958, and European Patent No. EP-B1-0045497. A composite spring comprising a cylinder of elastomeric material having metal annuli embedded therein at spaced apart locations is described in European Patent No. EP-A1-0155209.